The present invention generally relates to ceramic matrix composite (CMC) articles and processes for their production.
CMC materials have become of particular interest for use in turbomachinery as higher operating temperatures are sought to increase their efficiency. CMC materials, and particularly those proposed for gas turbine engine applications, typically comprise a ceramic fiber reinforcement material embedded in a ceramic matrix material. The reinforcement material serves as the load-bearing constituent of the CMC, and the ceramic matrix protects the reinforcement material, maintains the orientation of its fibers, and serves to dissipate loads to the reinforcement material.
Of particular interest to high-temperature applications are silicon-based composites, such as silicon carbide (SiC) as the matrix and/or reinforcement material. Notable examples of SiC/Si—SiC (fiber/matrix) CMC materials and processes are disclosed in commonly-assigned U.S. Pat. Nos. 5,015,540, 5,330,854, 5,336,350, 5,628,938, 6,024,898, 6,258,737, 6,403,158, and 6,503,441, and commonly-assigned U.S. Patent Application Publication No. 2004/0067316. One such process is known as “prepreg” melt-infiltration (MI), which in general terms entails the fabrication of CMCs using multiple prepreg layers, each in the form of a tape-like structure comprising the desired reinforcement material, a precursor of the CMC matrix material, binders, and other possible ingredients. The prepregs must undergo processing (including curing, also known as firing) to convert the precursor to the desired ceramic. Multiple plies of prepregs are stacked and debulked to form a laminate preform, a process referred to as “lay-up.” Following lay-up, the laminate preform will typically undergo debulking and curing while subjected to applied pressure and an elevated temperature, such as in an autoclave. The melt-infiltration process generally entails heating the laminate preform in a vacuum or an inert atmosphere to decompose (burnout) the binders and produce a porous preform ready for melt infiltration, after which the preform can be melt infiltrated with, for example, molten silicon supplied externally to the preform. The molten silicon infiltrates into the porosity and preferably reacts with constituents (for example, a carbon source) within the matrix to form a silicon-based ceramic (for example, silicon carbide) that fills the porosity to yield the desired CMC component.
CMC articles having inner cavities are desirable or necessary for some applications, including but not limited to cavities that define cooling slots/holes and complex cooling passages within airfoil components, as well as cavities intended to generally achieve weight reduction. Inner cavities can be produced in a CMC article by forming the laminate preform around a mandrel. However, the mandrels must be removed prior to melt infiltration. Mandrels that remain solid during burnout must be physically removed, which can be impossible if the desired cavity has twists or tapers. FIG. 1 schematically shows an example where a conventional steel mandrel 30 is intended to form a subsequent cavity in a section 20 of a laminate preform 10. The steel mandrel 30 cannot be removed from the preform 10 due to its being captured by a shoulder 22 defined by plies at one end of the preform 10. To address this issue, polymeric mandrels have been proposed that are formed of fugitive resins. Fugitive polymeric resins, in the context of this description, are typically hydro-carbon based solids which upon heating to a sufficiently high temperature, typically 400-800° C., volatilize leaving little or no carbon residue. Notable examples of fugitive resins include poly-methyl methacrylate and ply-vinyl alcohol. However, these resins have thermal expansion coefficients that may be five to ten times greater than the material of the CMC preform. The higher expansion coefficient of the fugitive resins can cause the CMC preform to distort during heating to decompose the binder resins. During burnout, the fugitive resins melt and the molten resin must be removed from the resultant cavity within the interior of the CMC article. Some of the molten resin may form a carbonaceous coating inside the cavity which, when reacted with silicon during subsequent melt infiltration, can alter the cavity dimensions. When using fugitive resins with larger-size CMC components, the amount of gases which must escape from or through the preform as the polymeric mandrel decomposes also increases. This necessitates using slower pyrolysis cycles which increases processing cycle time for the CMC components.
Accordingly, there is a need for improved methods capable of forming internal cavities within CMC articles.